Magnetic field is one of the most elementary and ubiquitous physical observables. Therefore, there are perennial needs for its detection with high sensitivity in a wide variety of applications ranging from nondestructive evaluation of integrated circuits to detection of brain activity signatures.
In the highest sensitivity limit, Superconducting Quantum Interference Devices (SQUIDs) have long served as the ultimate detectors with the sensitivity of ˜10−15 T/Hz1/2 operating at 4.2 K. Recently, a number of alternative techniques have been demonstrated including an atomic magnetometer with subfemtotesla resolution and magnetoresistive sensors with 32×10−15T (32 FT) sensitivity at 77 K. These techniques have significant drawbacks in that they require cryogenics and/or a cumbersome and unwieldy apparatus setup. From a practical and commercial point of view, these drawbacks inevitably translate to very expensive instruments or the tools not being suitable for everyday applications.
There is a perennial need to develop inexpensive high sensitivity magnetic field detectors to be used for a variety of applications including scanning magnetic probe microscopy. The ME effect devices may offer the solution for ˜10−12T sensitivity with a relatively simple setup in a room temperature environment.
It has been known to those skilled in the art that magnetostrictive materials may be utilized to perform relatively high sensitivity magnetic field detection (˜10−11 T/Hz1/2) at room temperature in a simple device configuration. Because many applications such as the detection of human brain activity (alpha waves), human heartbeat and inspection of electronic circuits do not require fT resolution, it is highly desirable to develop inexpensive 10−12 T (pT) resolution magnetometers which can be readily operated at room temperature.
The current surge of activity in multiferroic materials and structures are beginning to lead the way for the development of a new generation of magnetoelectric (ME) devices. Multiferroics are materials in which ferroelectricity and ferromagnetism coexist (Manfred Fiebig, “Revival of the magnetoelectric effect,” Journal of Physics D: Applied Physics 38, R123-R152 (2005). The structures in which conversion of magnetic field to electric field takes place through coupling of the magnetostrictive effect (of the ferromagnetic component) and the piezoelectric effect (of the ferroelectric component) are referred to herein as magnetoelectric (ME) devices.
The basic principle of the magnetoelectric effect is that the magnetic field induced strain in the magnetostrictive material is transferred to a strain in the piezoelectric material through elastic coupling which results in a piezo-induced voltage. While there are different types of multiferroic materials, by far the most promising structures for use as a magnetostrictive layer are the two-phase materials typically fabricated by coupling a magnetostrictive layer to a piezoelectric sample in a bulk configuration.
The “strength” of the ME effect in a multiferroic system can be characterized by the ME coefficient given by:αE=(∂ε/∂H)(∂E/∂ε)  (Eq. 1)where H is the applied magnetic field, ε is the induced strain, and E is the resulting induced electric field.
The ME coefficient αE carries information regarding the magnetostrictive coefficient of the magnetostrictive layer, the piezoelectric coefficient of the piezoelectric material as well as the elastic coupling between the two. While the first two are known and fixed for given materials, the elastic coupling may depend on the nature of the interface between the two layers as well as the geometry of the device. For a “perfect” coupling between two high coefficient materials, αE as high as ˜1 V/(cm Oe) has been predicted (S. Dong, J. Li, and D. Viehland, “Ultrahigh magnetic field sensitivity in laminates of TERFENOL-D and Pb(Mg1/3Nb2/3)O3—PbTiO3 crystal”, Appl. Phys. Lett., 83, 2265-2267 (2003).
One of the first such structures was demonstrated by simply “gluing” a layer of Terfenol-D((Tb,Dy)Fe2) to a ferroelectric polymer polyvinylidene fluoride (PVDF) where a significant ME signal was observed (K. Mori, and M. Wuttig, “Magnetoelectric coupling in Terfenol-D/polyvinylidenedifluoride composites”, Appl. Phys. Lett 81, 100-101 (2002). There have been reports of the ME effect in various multiferroic structures (G. Srinivasan, C. P. DeVreugd, M. I. Bichurin and V. M. Petrov, “Magnetoelectric interactions in bi-layers of yttrium iron garnet and lead magnesium niobate-lead titanate: Evidence for strong coupling in single crystals and epitaxial films”, Appl. Phys. Lett. 86, 222506-1-222506-3 (2005); G. Srinivasan, C. P. DeVreugd, C. S. Flattery, V. M. Laletsin, and N. Paddubnaya, “Magnetoelectric interactions in hot-pressed nickel zinc ferrite and lead zirconante titanate composites”, Appl. Phys. Lett. 85, 2550-2552 (2004); G. Srinivasan, C. P. DeVreugd, V. M. Laletsin, N. Paddubnay, M. I. Bichurin, V. M. Petrov and D. A. Filippov, “Resonant magnetoelectric coupling in trilayers of ferromagnetic alloys and piezoelectric lead zirconate titanate: the influienc of bias magnetic field”, Phys. Rev. B (Condensed Matter and Materials Physics) 71, 1844423-1-6 (2005); J. Ryu, Alfredo Vasques Carazo, Kenji Uchino and Hyoun-Ee Kim, “Magnetoelectric properties in piezoelectric and magnetostrictive laminate composites,” Jpn. J. Appl. Phys. 40, 4948-4951 (2001).
Recently an ME device was fabricated by sputtering a multilayer thin film of TbFe/FeCo on a bulk single crystal Pb(Mg1/3Nb2/3)O3—PbTiO3 (PMN-PT) to achieve a high ME signal (S. Stein, M. Wuttig, D. Viehland, and E. Quandt, “Magnetoelectric effect in sputtered composites”, J. Appl. Phys. 97, 1-1-1-3 (2005). There have also been reports of observation of the ME effect in composite structured multiferroic materials including the CoFe2O4—PbTiO3 epitaxial nanocomposite thin film (M. Murakami, K. -S. Chang, M. A. Aronova, C. -L. Lin, Ming H. Yu, J. Hattrick Simpers, M. Wuttig, I. Takeuchi, C. Gao, B. Hu, S. E. Lofland, L. A. Bedersky, “Tunable Multiferroic Properties in Nanocomposite PbTiO3—CoFe2O4 Epitaxial Thin Films,” Applied Physics Letters 87, 112901-1-3 (2005); C. Gao, Bo Hu, Xuefei Li, Chihui Liu, M. Murakami, K.-S. Chang, C. J. Long, M. Wuttig, and I. Takeuchi, “Measurement of the Magnetoelectric Coefficient using a Scanning Evanescent Microwave Microscope,” Appl. Phys. Lett. 87, 153505-1-153505-3 (2005).
A series of devices consisting of Terfenol-D and PMN-PT have been fabricated which demonstrated extremely high sensitivity (S. Dong, J. Cheng, J. Li, and D. Viehland, “Enhanced magnetoelectric effects in laminate composites of TERFENOL-D/Pb(Zr, Ti)O3 under resonant drive”, Appl. Phys. Lett. 83, 4812-4814 (2003); S. Dong, J. Bai, J. Zhai, J. Li, J. Lu, and D. Viehland, “Circumferential-mode, quasi-ring-type, magnetoelectric laminate composite—a highly sensitive electric current and/or vortex magnetic field sensor”, Appl. Phys. Lett., 86, 182506-1-182506-3 (2005); S. Dong, J. Zhai, J. Bai, J. Li, and D. Viehland, “Push-pull mode magnetostrictive/piezoelectric laminate composite with an enhanced magnetoelectric voltage coefficient”, Appl. Phys. Lett., 87, 062502-1-062502-3 (2005). By operating the device at the mechanical resonant frequency, sensitivity as high as ˜1 pT has been achieved. The observed ME coefficient typically ranges from ˜mV/(cm Oe) to ˜1 V/(cm Oe), with the high end being mostly observed under resonant conditions.
These devices are typically operated while they are biased with an external DC field so that the ME response is at its maximum. This is typically at the point in the magnetization-field (M-H) hysteresis curve where the susceptibility (derivative of the M-H curve) is maximum. The field to be detected is typically an AC field which modulates the response at the maximum susceptibility point. For optimum performance, the magnetic field is applied in parallel to the pre-magnetized direction of the magnetostrictive layer which is usually in the in-plane direction.
FIG. 1 schematically illustrates a common configuration of the ME device which has a sandwich structure in which two metallic magnetostrictive materials 10 sandwich the piezoelectric layer 12 therebetween and are also used as the electrodes to monitor the piezo-induced voltage. In this configuration, the strain induced in the piezoelectric layer 12 is converted to the voltage through the ME coefficient. The ME voltage is detected using a lock-in amplifier. The device can detect a field on the order of 10−12T.
Disadvantageously, high sensitivity ME devices demonstrated to date have been fabricated using bulk or hybrid laminate materials and are typically mm˜cm in dimensions. In order to pursue their implementation in microelectronics and integration with other circuit components, it is desirable to fabricate all thin film based devices. However, due to the fact that ME devices rely on their layers being able to display mechanical flexibility, one disadvantage of thin film structures is that they inevitably have to be deposited on substrates thus being exposed to the substrate clamping effect which deteriorates the performance characteristics of ME devices.
The use of micromachined cantilevers allows thin films to exhibit some mechanical “freedom” and reduce the clamping effect caused by the substrate. Fabrication and utility of PZT (lead zirconium titanate) cantilever structures have been demonstrated by a number of groups (Joon-Shik Park, Hyo-Derk Park, Sung-Goon Kang, “Fabrication and properties of PZT micro cantilevers using isotropic silicon dry etching process by XeF2 gas for release process,” Sensors and Actuators, A 117, 1-7 (2005); Ghi Yuun Kang, Sang-Woo Bae, Hyung-Ho Park, and Tae Song Kim, “Fabrication and electromechanical properties of a self-actuating Pb(Zr0.52 Ti0.48)O3 microcantilever using a direct patternable sol-gel method,” Appl. Phys. Lett. 88, 042904-1-3 (2006); B. Piekarski, Ph.D. thesis, “Lead zirconate titanate thin films for piezoelectric actuation and sensing of MEMS resonators,” University of Maryland (2005).
The above publications reported the use of PZT on cantilevers. Cantilevers have also been used to characterize mechanical properties of thin films of elastic materials such as shape memory alloys and ferromagnetic shape memory alloys (S. A. Mathews, Manfred Wuttig and Quanmin Su, “The Effect of Substrate Constraint on the Martensitic Transformation of Ni—Ti Thin Films,” Met. Trans. 27A, 2859 (1996); Quanmin Su, J. Morillo, Y. Wen and Manfred Wuttig, “Young's Modulus of Amorphous Terfenol-D Thin Films,” J. Appl. Phys. 80, 3604-3606 (1996); Quanmin Su, Yun Zheng and Manfred Wuttig, “Graphoepitaxial Shape Memory Thin Films on Si,” Appl. Phys. Lett, 73, 750-752 (1998); J. Morillo, Quanmin Su, Don Novotny and Manfred Wuttig, “Micromachined silicon torsional resonator for magnetic anistotropy measurement,” Rev. Sci. Instrum. 69, number 11, 3908-3912 (1998); M. Wuttig, “Thin film SMA/Si composite actuators,” Proc. SPIE-Int. Soc. Opt. Eng. (USA), Proceedings of the SPIE—The International Society for Optical Engineering, vol. 3984 p. 450-5; O. O. Famodu, J. Hattrick-Simpers, M. Aronova, K.-S. Chang, M. Murakami, M. Wuttig, T. Okazaki, Y. Furuya and I. Takeuchi, “Combinatorial Investigation of Ferromagnetic Shape-Memory Alloys in the Ni—Mn—Al Ternary system using a Composition Spread Technique,” Materials Transactions, JIM, 45, 173-177 (2004); I. Takeuchi, O. Famodu, J. C. Rad, M. Aronova, K.-S. Chang, C. Craciunescu, S. E. Lofland, M. Wuttig, F. C. Wellstood, L. Knouse, A. Orozco, “Identification of Novel Compositions of Ferromagnetic Shape Memory Alloys using Composition Spreads,” Nature Materials 2, 180-184 (2003).
Despite the previous developments in the field of thin film magnetoelectric materials, such have been supported by substrates thus inevitably suffering, through the substrate clamping effect, a deterioration in their coupling characteristics. No attempt has been done to avoid the substrate clamping effect. A magnetoelectric device built on a cantilever has not been suggested or developed in attempt to improve the performance of the thin film ME devices. Also, to date, no device has been developed or demonstrated, where all active layers were made of thin films. Piezoresistive layer has also never been used in conjunction with a magnetostrictive layer to fabricate a magnetoelectric structure.
It is therefore desirable to use advantages of thin film technology, as well as benefits allowed by cantilever structures, in a new magnetoelectric device with superior operational characteristics.